1. Field of the Invention
The present invention relates generally to angular position sensors and, more particularly, to an angular position sensor which utilizes a rotatable magnet in cooperation with a magnetically sensitive device to identify the angular position of a rotatable shaft.
2. Description of the Prior Art
Many types of angular position sensors are known to those skilled in the art. Some of these position sensors utilize magnetically sensitive devices in association with rotatable magnets. In some cases, the magnetically sensitive device is a Hall effect element.
U.S. Pat. No. 4,406,272, which issued to Kiess et al on Sep. 27, 1983, discloses a magnetic sensor for use in an ignition system of an automobile. The sensor is useable in an internal combustion engine of the automobile and disposes a Hall effect device between a pair of opposing permanent magnets for concurrently generating dual magnetic flux fields within respective airgap regions formed between each of the magnets and the device. A toothed disc rotatably connected to the crank shaft of the engine causes different teeth to shunt the field in each of the regions in a predetermined sequence for generating pulses at the device output indicative of the firing order of the engine.
U.S. Pat. No. 4,373,486, which issued to Nichols et al on Feb. 15, 1983, describes a rotational position and velocity sensing apparatus in which a ferrous disc is rotatable driven by an internal combustion engine shaft. The disc is provided with inner and outer circular rims that project outwardly from one side of the disc. The outer rim has two arcuate notches of predetermined arcuate length and position and the inner rim has three arcuate notches of predetermined arcuate length and position. A permanent magnet is mounted between and radially spaced from two Hall effect sensor devices in fixed relation to the shaft axis such that the outer rim passes between one Hall effect device and the magnet and the second rim passes between the second Hall effect device and the magnet as the shaft is rotated about its central axis.
U.S. Pat. No. 4,471,652, which issued to Yasuhara et al on Sep. 18, 1984, describes a position detector for a rotary element in which a permanent magnet is mounted on the rotary member by a nonmagnetic holder that magnetically and thermally insulates the magnet from the rotary member for the purpose of prolonging the life of the magnet and obviating the noise that is normally produced by a stationary pickup that is excited by the passing magnet.
U.S. Pat. No. 4,381,506, which issued to Linn et al on Apr. 26, 1983, discloses a position signal transducer which provides a vibration and shock insensitive apparatus by winding a coil of a lamella-like carrier which has a body element of electrically insulating material on which a foil of magnetically highly permeable material is positioned on at least one surface. The lamella-like foil combinations can be stacked or, alternatively, a unitary body with foils on both sides can be provided. A short circuit ring, which is moveable along the core, changes the inductance of the coil wound thereon.
U.S. Pat. No. 4,789,826, which issued to Willett on Dec. 6, 1988, describes a system for sensing the angular position of the rotatable member by using a Hall effect transducer. The device senses the angular rotation of a member, such as the shaft of a tension arm assembly, by the combination of a circular type of magnet secured to the rotatable member and selectively polarized relative to its diameter to define a magnetically North and magnetically South pole pair. It also comprises a stationary Hall effect transducing device that is secured in close and constant proximity to the ring magnet. A circuit coupled to the transducing device includes offset and amplification stages. The Hall effect transducing device is located in the region of a magnetic null of the field generated by the magnetic poles when the rotatable member is in a selected angular position.
U.S. Pat. No. 4,785,242, which issued to Vaidya et al on Nov. 15, 1988, describes a position detecting apparatus that uses multiple magnetic sensors for determining relative and absolute angular positions. The apparatus utilizes a first magnetic sensing device for accurately determining the angular position of a rotor and a second magnetic sensing device for absolutely determining the angular position of the rotor. The first sensing device includes a first target operatively associated with the rotor so as to rotate therewith and also incudes a first sensor disposed at a fixed distance from the first target independent of the absolute angular position of the rotor to define a first airgap therebetween and a first magnet is disposed in proximity to the first target and the first sensor to create a first magnetic field in the first airgap.
U.S. Pat. No. 4,677,377, which issued to Takahashi et al on Jun. 30, 1987, discloses a position detecting sensor for detecting the position of a moving object. It comprises a magnetic recording medium carried on a moving object and having a signal track recording a magnetic signal, a first MR element disposed near the magnetic recording medium and varying its internal resistance by sensing the magnetism of the magnetically recording medium and a magnetic sensor electrically picking up resistance variations of the MR element and detecting the magnetic signal of the magnetic recording medium.
An article titled "Annular Magnet Position Sensor", by G. Lemarquand and V. Lemarquand, which appeared in the IEEE Transactions on Magnetics, Volume 26, Number 5 in Sep. 1990, describes an absolute permanent magnet angular position sensor which utilizes two magnetically sensitive devices. The geometry of the device was optimized to obtain a perfectly linear variation law of the normal induction in the air gap. The sensor is characterized by a simple electronic treatment, a low realization cost and a resolution of approximately a tenth of a degree. This sensor utilizes a shaped magnet disposed around a core rotor and disposed within a stator that is made of a magnetic material. Two magnetically sensitive devices, such as Hall effect generators, are arranged between the rotatable magnet and the stator at different positions to enable associated electronic circuitry to accurately determine the angular position of the stator as a function of the signals received from the two Hall effect devices.
U.S. Pat. No. 4,570,118, which issued to Tomczak on Feb. 11, 1986, describes an angular position transducer which includes permanent magnets and a Hall effect device. It provides a transducer for creating an electrical signal which is proportional to the angular position of a member that is pivotally mounted on a given axis. It comprises an element pivoted directly by the member and containing a means for creating a flux field linearly varying intensity along a given operating line extending in an airgap between spaced portions and having a preselected arcuate shape. A linear Hall effect device, with an output voltage that is proportional to the intensity of the flux field to which the device is disposed, is mounted at a fixed position on the operating line and within the airgap. Consequently, as the element is pivoted by the monitored member, the output voltage from the Hall effect device varies proportionally to the position of the Hall effect device along the operating line.
Position sensing devices that utilize a magnet and a magnetically sensitive device, such as a Hall effect device, suffer from a common disability because the relative position of the Hall effect element with respect to the magnet is critical to the proper operation of the apparatus and even minor deviations in that distance can adversely affect the accuracy of the device. Since the magnet is most commonly caused to rotate in an apparatus of this kind, a shaft is generally used and a bearing is required to support the shaft for rotation about a central axis. If the relative sizes of the shaft and its bearing are not accurately maintained, the shaft can assume varying positions relative to the bearing and the magnet can therefore assume variable positions relative to the magnetically sensitive device. Because of the inverse square relationship between the distance that a Hall effect device is offset from a magnet and the magnetic strength imposed on the Hall effect device, relatively slight deviations in the position of the magnet have significant deleterious results on the accuracy of the system because of the unpredictable strength of the magnetic field at the position of the Hall effect element. It would therefore be beneficial if a angular position sensor can be constructed in such a way that the magnetically sensitive device is relatively insensitive to minor changes in the position of the rotatable magnet in an angular position sensor.